From what I can gather, there's no real gain to fitting one apart from the fact that it looks good under the bonnet and the DV / induction noise might be slightly amplified. I'd be careful about what you choose, as there's quite a divide between people who swear by them, and people who report problems such as oil from the filter depositing on the MAF (not good) and unwanted intake of warm air from the engine area.
If you're running immense boost, it may be beneficial, as a huge turbo needs to be fed with as much of a charge as possible to do it's job effectively. Every single big BHP Cossie I've ever seen is running a ****** great cone.
The theory of induction kits is not a bad one, but in practise you may find that you're not gaining any noticable power, and may even lose it.
My advise if you're dead set on getting one, is to look for a stainless mesh design, ie. one that doesn't use oil on the filter material. (I've seen a couple of threads one here discussing this). And also think carefully about shielding the unit from any heat dispursion - an induction kit is pointless unless it sucks COLD air in.
I've seen a few aftermarket airboxes that claim to achieve incresed airflow. These are sealed units, with a high-flow element that are designed specifically to shield any warm air, but still improve on the factory unit. I think there's one that APR/Awesome GTI do. They are alot of money though!
Ask yourself why you want one, whether it's for a bit more of a whoosh from the engine, or an extra 0.0004 BHP, then weigh up the pros and cons from there.
Let me know how you get on, as I'd personally never rule out any modification until I've tried it.
Give it a go, but don't chuck your stock airbox away as you never know....
Andrew.
